[Q] [I9001] Weak WiFi signal

I just got a second hand Galaxy S Plus (I9001) but the wifi is very poor. The router (Apple Airport Extreme) is about 10 meters away with no obstacles. Depending on the rom the signal strength is between -70 and -80 dBm.

The strange thing is that with the original rom that was released in the Netherlands based on Android 2.3.5 the wifi is really bad:

While with another official rom based on 2.3.6 the wifi is still bad, but a little better then the above:

And again with a custom rom (CM 10.1 as well as CM 11.0) the wifi is extremely poor:

Can somebody shed some light on this? I would like to use CM 10.1 or CM 11.0, but the wifi just becomes unusable.

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